1. Field of the Invention
The present invention relates to a method of producing metal quantum dots. In particular the method enables metal quantum dots of uniform size to be produced which can be used in providing very regular metal layers.
2. Discussion of Prior Art
Quantum dots are defined as small particles whose linear dimension in all three directions is less than the de Broglie wavelength of the electrons or holes. Such particles can have a greatly modified electronic structure from the corresponding bulk material. To date methods of producing quantum dots have focussed on semiconductor material for use in the field of optoelectronics. A paper describing semiconductor quantum dots and some of their properties has been published in Angewandte Chemie International Edition (English) 1993, 32, at pages 41-53: "semiconductor q-particles: chemistry in the transition region between solid state and molecules" by Horst Weller. A number of methods of producing semiconductor quantum dots have also been tried and these have been centred on the generation of colloids or inverse micelles or on "smokes". Variability in size has though remained a problem. More recently a method of producing semiconductor quantum dots using an electrostatic nozzle assembly has been developed in which each of the individual droplets formed in an aerosol functions as a separate reaction chamber in the presence of a gas phase reagent.
With the present invention there has been the surprising discovery that single element metal quantum dots may be produced using an electrostatic nozzle and irradiation of the aerosol formed in the absence of a gas phase reagent.
Thus, the present invention provides a method of producing metal quantum dots, which method comprises providing a solution in an evaporable solvent of the metal in chemically combined form, passing the solution through an electrostatic capillary nozzle to form droplets of the solution in a chamber, irradiation of the droplets within the chamber with photons and removing the evaporable solvent from the droplets to form metal particles.
The method starts with a solution in an evaporable solvent of the chosen metal in chemically combined form. The evaporable solvent is volatile in the sense that the solvent is evaporable under the reaction conditions used and may be water or an organic solvent whose nature is not important. Although other chemically combined forms are possible, the metal is typically present in the form of a salt, preferably a salt with a volatile anion such as nitrate or chloride. A wide variety of metals may be used indeed almost any metal which produces positive ions in solution such as Ag, Ni, Fe or Co.
The solution may contain polyphosphate as stabiliser, for example in the form of sodium polyphosphate which has an average chain length of about 15 PO.sup.-.sub.3 units. Polyphosphate is well suited for the stabilisation of nanometer size particles, because the chain is strongly bound by metal ions on to the particle surface. It causes electrostatic repulsion between particles because of its charge, and also keeps them apart sterically because of its chain length. Other frequently used stabilisers are thiols. Alternatively, the starting solution may contain an organic polymer which encapsulates the metal particle. Many organic polymers, in solution or dispersion in the volatile liquid, are suitable and known to those working in the field. Examples of suitable polymers include polyvinyl alcohol, polyvinyl acetate, polymethyl methacrylate and polycarbonate. With each of the above examples of stabilisers, particle aggregation whilst in solution is prevented. Subsequently though the polyphosphate or organic polymer binds the dried particles into a film.
The solution is converted into droplets, and ideally the size of these droplets is made as nearly uniform as possible. For this purpose an electrostatic capillary nozzle is used in which a jet of aerosol droplets can be formed by the electrostatic deformation of a meniscus of the starting solution. The droplet size can be controlled by varying the flow rate from a reservoir and the voltage applied to the nozzle.
The droplets emerge from the nozzle into a chamber preferably under low-pressure conditions and are irradiated preferably using a laser. The volatile solvent is evaporated off to leave the desired particles of metal. Where the starting solution contained a polymer, the particles are encapsulated in the polymer.
The size of the resulting particles or quantum dots depends on two factors: the concentration of the chemically combined metal in the starting solution; and the size of the formed droplets. Both these variables are readily controlled and pre- determined either experimentally or theoretically to provide quantum dots of desired size, which is typically less than 25 nm, and preferably in the range 1 to 20 nm.